The present invention relates generally to apparatus for the detection and measurement of magnetic anomalies in magnetic materials and is more particularly directed to apparatus for detecting magnetic flux leakage such as used for inspecting for defects in the bottoms of chemical and petroleum storage tanks or in tubing such as used in oil and gas wells.
The problems of detecting defects in ferromagnetic materials may be illustrated with reference to chemical and petroleum storage tanks. These tanks face gradual and continual deterioration due to the harsh chemical environment both outside and within the vessel. The steel walls of a tank are subject to corrosion, pitting, and other chemical and physical processes that can cause localized damage to the walls. Such localized damaged regions can develop into leaks or in extreme cases can lead to rupture of the tank. The tank bottom is exposed to corrosion or similar damage from the underside as well as from the top side. A tank typically rests on sand, gravel, crushed limestone, clay or similar base of varied composition. When the tank is filled, the bottom flexes and presses into the material under the weight of the contents. When the tank is then emptied, the bottom rises causing air and moisture to be drawn in, which accelerates the underside deterioration process.
To guard against environmentally damaging leaks or other tank failure, tank bottoms should be inspected periodically for early signs of damage conditions that may result in leakage. The underside of the tank bottom of course is inaccessible and thus cannot be inspected directly. One popular form of inspection apparatus looks for magnetic anomalies caused by local damage to the steel tank bottom. This apparatus includes one or more strong permanent magnets or electromagnets that induce a magnetic field within the steel plate forming the tank bottom that in effect locally magnetizes the plate. When the local region of the plate under the magnet is free of defects, it produces an induced magnetic flux of a known form that is highly regular. Localized defects from corrosion, pitting and the like produce irregularities in the highly regular form of the flux pattern that "leak out" of the steel plate. The irregularities in the otherwise regular flux pattern may be detected by sensors in the inspection apparatus positioned just above the plate surface, and this is so even if the defect producing the magnetic anomaly is on the inaccessible underside of the plate. In this way, detecting a magnetic anomaly signals the site of a possible defect in the steel bottom. Such apparatus is disclosed, for example, in U.S. Pat. No. 4,814,705 of Saunderson.
Although magnetic flux leakage methods have proved useful for detecting the presence of magnetic anomalies, the known magnetic flux leakage inspection devices intended for use in the field are not very precise. It has turned out to be difficult, cumbersome or expensive to apply magnetic flux leakage methods in the field for determining the detailed characteristics of magnetic anomalies with any quantitative precision. It would of course be useful to make precision measurements of detected anomalies to help in evaluating the nature of the defect and extent of the damage. The signals generated in the sensors by the flux leakage from an anomaly, however, are generally weak and can easily be obscured by spurious signals from the regular, i.e., non-anomalous, magnetic pattern. Moreover, maintaining the calibration of the inspection apparatus sufficiently well for absolute measurements throughout the course of an inspection has turned out to be troublesome.
Typical approaches to quantitative measurements in the past have relied instead on comparison of the measured response with the results of an essentially identical measurement on a specially prepared calibration specimen formed of a known material and containing magnetic anomalies of known features. Here the inspection instrument is calibrated (usually before each inspection session) with a specimen containing magnetic anomalies with features bracketing those expected in the material being evaluated. This procedure imposes a practical limitation in applying the magnetic flux leakage technique. A concomitant drawback is the cost in time, material, and handling associated with purchasing and maintaining a set of calibration specimens and performing the frequent calibration operations. To avoid these problems, magnetic flux leakage inspection apparatus has sometimes merely been used for preliminary screening to locate magnetic anomalies of possible significance. The located anomalies have then been subjected to a more accurate, but more time-consuming ultrasonic mapping technique to determine their characteristics for purposes of assessing the damage to the tank bottom.
Similar problems also arise in inspecting pipe and tubing used in the drilling, completion and production of oil and gas wells. Here strings of tubular sections are connected together to form an extended length for such purposes as drilling, casing, or transmission between the well head and a downhole location. The tubing is subject to much the same sort of mechanical damage or corrosion pitting as the storage tanks discussed above. Magnetic flux leakage techniques have been applied here too for the detection and evaluation of magnetic anomalies indicative of such damage and are disclosed, for example, in U.S. Pat. No. 4,704,580 of Moake et al. and U.S. Pat. No. 4,710,712 of Bradfield et al.